FIG. 1 shows one embodiment of the present invention employing an interferometer in accordance with the teaching of said copending application. In said copending application, reference is made to electronic processing of the interference signals obtained from the interference correlation microscope to provide images of the surface Briefly, an interference signal is generated for each x,y position of the surface as the surface is scanned in the z direction.
Unlike a standard microscope or a confocal microscope, the images produced by an interference microscope are extremely hard to interpret due to the presence of bright and dark fringes. The object of this invention is to provide a method of processing the signals to extract useful data from the interference images. There are already some existing algorithms for extracting phase information used by commercial firms. For example, phase shifting techniques are employed by both the Wygo and Zygo Corporations to extract phase data from the interference images. By employing such algorithms, surface profiles in the nanometer range have been measured. However, the major difficulty associated with such phase shifting techniques is to get the proper phase shift between the data frames. It is impossible to determine the exact phase shift between data frames with an illumination source that is wider band than a laser source. Another drawback of such algorithms is that they can only be used to measure the surface profiles of opaque objects.
The present invention is directed to a process capable of removing the aliasing or the fringes in the images. This is achieved by filtering the data along the axial axis or the z-axis. Hence, the object is scanned axially along the z-axis by a piezoelectric pusher in equal discrete steps and a CCD camera is synchronized by computer control to acquire an image of the object at each step. The acquired images are then stacked inside the computer for further signal processing. One method for extracting the amplitude and the phase information from the interference images is based on filtering the data in the spatial frequency domain. Each pixel (x,y) in the image plane is Fourier transformed along the z-axis, filtered in the frequency domain and then inverse transformed back to the space domain. This algorithm is powerful because it can produce two-dimensional cross-sectional amplitude and phase images of any objects. However, if the number of pixels in the image is large, this process can be computationally intensive and slow. Consequently, it is mainly reserved for producing line scans in critical dimension measurements.
Another method is based on the Hilbert transform and is a more efficient procedure for obtaining two-dimensional cross-sectional images. The simplicity of the Hilbert filter coefficients allows the filtering to be carried out in the space domain with a frame grabber alone. Since no Fourier transforms are required, the number of computations is greatly reduced. Moreover, since a frame grabber operates on a frame-by-frame basis, a significant amount of computation time can be saved.